Radio frequency (RF) communication and, more specifically, high-speed wireless network communication in cellular baseband frequencies are ubiquitous in today's consumer electronic devices. Wireless terminals include not only cellular phones, but other consumer electronic devices that include chipsets that implement wireless transmitters and/or receivers. Long-Term Evolution (LTE) is a standard for high speed wireless communication, and many chipsets are designed to meet the LTE standard. The counter inter-modulation (CIM) performance of the wireless terminal transmitter path is a key parameter for the design of these devices. In order to achieve a low CIM distortion level, calibration of the transmitter is often required, especially for a multi-phase mixer transceiver architecture.
FIG. 1 illustrates a wireless communications system 100, in accordance with the prior art. As shown in FIG. 1, the wireless communications system 100 includes a baseband integrated circuit and transceiver 110 that includes a transmitter (TX) 120, a local oscillator 130, and a measurement receiver (MRX) 140. The transmitter 120 generates an output signal that is coupled to a power amplifier 160. The power amplifier 160 generates an amplified signal that is passed through a bandpass filter 170, a coupler 180, and one or more antenna 190. The coupler 180 couples the signal output from the bandpass filter 170 to a feedback signal that is connected to the input of the measurement receiver 140. In effect, the feedback signal is the same as the filtered, amplified signal transmitted wirelessly by the antenna 190.
Calibration of the transmitter 120 is typically performed using the measurement receiver 140, which is configured to measure the RF signal generated by the transmitter 120. Analysis of the measured signal may be performed to determine adjustments for one or more parameters of the transmitter that affect the CIM distortion level. One parameter that is often adjusted is a duty cycle of the RF signals generated by the local oscillator 130 that feeds an N-phase mixer in the transmitter 120. By varying the one or more parameters and measuring a CIM distortion level of the output signal, the measurement receiver 140 may be utilized to calibrate the transmitter 120 by choosing parameter values that provide an optimum reduction in CIM distortion in the output signal.
Unfortunately, the measurement receiver path may also create CIM distortion that is added to the CIM distortion from the transmitter during the analysis. The CIM distortion from the measurement receiver path may come from various sources including but not limited to, e.g., power amplifier harmonics mixing with the measurement receiver local oscillator signal or harmonic distortion from an analog-to-digital converter (ADC) that converts the feedback signal to digital samples. The CIM distortion generated along the measurement receiver path from the coupler 180 to the measurement receiver 140 can affect the analysis of the feedback signal such that the parameter values selected during calibration do not minimize the CIM distortion of the transmitter 120. There are various work arounds to this issue. For example, the voltage controlled oscillator clock speed may be increased, but this may increase power consumption of the system. An additional low pass filter between the coupler 180 and the measurement receiver 140 may attenuate certain CIM distortion that negatively affects calibration, and the design of the measurement receiver may be ultra linear to minimize CIM distortion from the measurement receiver, which may be achieved at the cost of more complicated circuit design. Thus, there is a need for addressing this issue and/or other issues associated with the prior art.